Medium voltage power cables (5 kV to 46 kV) must meet various safety standards set by the cable standard organizations. Such standards include AEIC CS 8 (Association of Edison Illuminating Companies), ICEA S-94-649 (Insulated Cable Engineering Association, CSA C68.3 (Canadian Standards Association) and UL 1072 (Underwriters Laboratories)
Additional standards may be applied when these power cables are made water blocked. Such cables must meet additional standards, such as ICEA T-31-610 which is a test for resistance for longitudinal water penetration.
In the prior art “strandblock” has been used to meet these standards. Strandblock is essentially a process whereby a strand filling compound, such as a gel or other water blocking agent is applied between the gaps and spaces between individual elements of a conductor core, blocking the penetration of water. Although many improvements have been made over the years, Strandblock remains a specialized slow and unclean process which is costly and difficult to maintain.
Water swellable powders are a form of polymers that, when brought into contact with water, expand, trapping the water within, forming a water locking gel-like substance. One example of a water swellable powder is formed with polymers that are in the form of polymeric chains coiled, and lined with carboxyl groups. When these water swellable powders are wetted, the carboxyl groups change to be negatively charged, forcing the chains to “uncoil,” resulting in a rapidly forming water absorbing gel.
Water swellable powders of this type are commercially available for use in the cable industry. One form for such powders is to be applied to yarns or tapes impregnated with the powders. These yarns and tapes may be applied at various stages during the cable construction.
For example, a typical prior art cable as shown in FIG. 1 includes a cable core of aluminum alloy or copper conductor elements (strands, wires), which are assembled to form a conductor. Around this conductor, a layer of insulating material, such as polyethylene (PE) is applied. In the case of medium and high voltage cables, this layer of insulating material is applied between two layers of semi-conductor material, such as polyolefin with carbon black. The cable may contain other cable components such as metallic screens, armour, jacket etc.
In one prior art arrangement as shown in CA 2 394 846, a water swellable tape is applied around the external copper screen, under the jacket so as to form an improved water blocking structure.
In another prior art arrangement, water blocking yarns may be fashioned for insertion within the conductor itself for use as a water protection measure. However, the simple application of water swellable yarns to a conductor core does not necessarily provide adequate water protection to meet the necessary safety standards such as those met by the strandblock method outlined above.